The exploitation of undersea hydrocarbon deposits is usually carried out from installations situated above sea level and supported by fixed platforms.
In zones where the depth of water is less than 300 m, the working loads are supported by relatively rigid platforms, than the periods shorter of the swell, in the order of 5 seconds maximum.
The construction of such platforms in water depths greater than 300 m leads to structures having prohibitive weights.
For great depths, structures which are flexible with respect to horizontal deformations, i.e. having natural bending periods greater than the period of the swell, have been envisaged and have also been installed.
The dynamic deformations of a structure are formed by the combination of different modes of deformation inherent to the structure, called natural modes. With each natural mode is associated a period called natural period of the structure. There are natural bending moves for the horizontal movements, twisting modes for rotation around a vertical axis and other modes which relate to vertical movements. The first and second natural bending modes correspond to the highest natural bending periods. For an exciting force with period equal to a natural period of the structure, the trend of the deformation will be very close to that of the corresponding mode and for an exciting force whose period is for example between the first two natural bending periods, the movement will be mainly a composition of the oscillations of the first two bending modes.
The dynamic behavior of a structure excited by a periodic force with a period shorter than the natural period of the structure, is such that the movement of the structure is in phase opposition with the exciting forces. Thus, the inertial forces which are equal to the product of the mass multiplied by the acceleration of the structure with a sign change, are in phase opposition with the exciting forces. The internal stresses induced in the structure, which are the resultants of the exciting forces and inertial forces, are then less than the exciting forces if the natural period of the structure is greater than the exciting periods and sufficiently removed therefrom, for example a natural period in the order of twice that on the exciting periods.
Among the structures which have been contemplated and even installed, are oscillating platforms and guyed platforms.
These platforms are made sufficiently flexible by incorporating into the structure a very flexible element, even including a hinged connection (French Pat. No. 82 12775 of July 22, 1982). In all cases the flexible element is localized. It then occurs that this element can only transmit extremely limited stresses in so far as bending and twisting are concerned.
The reaction forces with respect to the bending induced by the horizontal loads generated by the swell, the current and the wind reaction are provided either by a buoyancy reserve or by guys; the reaction forces due to the structure itself remain small.
The twisting loads which cannot be absorbed by the structure, taking into account the "flexible section", must be absorbed either by guys or by other elements specially designed for this purpose.
The localization of the flexible zone implies considerable deformations in the area of this zone. These deformations are not generally compatible with what is admissible for well conductor pipes and are therefore the cause of difficulties in fixing these offshore pipes.